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As I asked in "Adding multiple analog signal" before, I got that: for adding some signal together, we can only use op-amp, instead of lovely integrated circuit (ough). But the question arise because now I want to add differential signal output from ADA8282 and I have found this ST application note AN4586, "Signal conditioning, differential to single-ended amplification", helpful for amplifying differential to single ended, but is there any easier way than converting each one separately to single ended, then add them together? I really hate handling op-amp by two power supply(ough!).

I have found book about this: enter image description here It tell: maximum feedback resistor is restricted to input offset voltage. I want to minimize it to minimize OpAmp noise, (2) What is minimum limit?

Other methods:

(3) Can I use this method (serializing sources) that is easier because we don't have to waste anything to drive OpAmp: enter image description here Not: (4) after this I want to send signal through balanced line.

Why I want to add 4ch together? I don't want to load them on processor then I want it to be analog. These are come from 4 antenna that placed close to one another, after some analog process we have pure sine wave range from 0 to 3Mhz, I want to add these 4 channel to improve SNR (ADA8282 told something about phase matching then I hope summed signal will have better SNR). Also if I decided to change LNA PGAs gain they will all be changed together.

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  • \$\begingroup\$ You seem to have an aversion to OP-amps. Remember that they don't know or care if you have one or two or three power supplies. I've used one with a single 2.4 volt supply. You don't need a negative voltage. \$\endgroup\$
    – pipe
    Commented Nov 22, 2018 at 14:44
  • \$\begingroup\$ @pipe I feel so unsure. I want a board, as compact as possible. But good conversion need 2 OpAmp just as balun, totally just 9 OpAmp for adding 4ch of diff. sig. horrible. \$\endgroup\$
    – mohammadsdtmnd
    Commented Nov 24, 2018 at 6:55
  • \$\begingroup\$ @mohammadsdtmnd is there some clever idea about adding the outputs of these 4 channels together? You will end up with a mix of signals that can be somewhat focussed towards one signal or the other by using the PGAs to set different gain levels but why would you want to do this or maybe I'm missing what you are trying to achieve? \$\endgroup\$
    – Andy aka
    Commented Dec 14, 2018 at 14:15
  • \$\begingroup\$ @Andyaka Do yo mean: Why yo want to add them? is they are all one signal or what it is? Ok I've update the question. \$\endgroup\$
    – mohammadsdtmnd
    Commented Dec 14, 2018 at 14:31
  • \$\begingroup\$ OK I see. You'll get a 6 dB SNR improvement. \$\endgroup\$
    – Andy aka
    Commented Dec 14, 2018 at 14:36

1 Answer 1

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... we can only use op-amp, instead of lovely integrated circuit (ough).

An op-amp is nowadays almost always an integrated circuit. Be glad you don't have to make them out of transistors!

enter image description here

Figure 1. A summing differential amplifier using only one op-amp. Source Section G10: Differential Summing by UNCC.

The article linked above discusses the maths in some detail. I haven't read it through but I suspect that you need to be aware of limitations of the circuit due to possible degradation of common-mode rejection ratio (CMMR) or offsets caused by bias currents.

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  • \$\begingroup\$ It's nice but, I think, though it's only correct way, routing on PCB for 4 differential channel is really complicated. Also I searched your key-word "differential amplifier", and I've found lots of thing like the picture you posted, thanks. do u have any sug. for it's routing problem? \$\endgroup\$
    – mohammadsdtmnd
    Commented Nov 24, 2018 at 16:13
  • \$\begingroup\$ What are bias currents in this image? \$\endgroup\$
    – mohammadsdtmnd
    Commented Nov 25, 2018 at 11:17
  • \$\begingroup\$ They are the input bias or leakage currents which will be specified in your chosen op-amp datasheet. \$\endgroup\$
    – Transistor
    Commented Nov 25, 2018 at 12:52
  • \$\begingroup\$ What's about making sources series? isn't that easier? \$\endgroup\$
    – mohammadsdtmnd
    Commented Dec 14, 2018 at 13:21
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    \$\begingroup\$ I object!!!! There is absolutely nothing wrong with making op Amos out of transistors!!! Particularly if you are an IC designer. \$\endgroup\$
    – Edgar Brown
    Commented Dec 14, 2018 at 13:32

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